1. Field of the Invention
The present invention relates to a suspension device for a vehicle.
2. Description of the Related Art
A suspension device in a vehicle, such as an automobile, is usually provided with a shock absorber to prevent the transmission of vibration occurring at the wheels in accordance with road conditions to the vehicle body. Such a shock absorber is usually arranged between the wheel and a vehicle body, and comprises a spring and a damper arranged in a parallel relationship to each other.
In such a type of shock absorber, a shock absorbing operation is controlled by selecting the value of the spring coefficient k of the spring and damping coefficient c of the damper with respect to the value of the weight m of the vehicle, in such a manner that vibration at the wheels is dampened in accordance with damping ratio .zeta.=c/(2.sqroot.mxk).
The prior art shock absorber suffers from a drawback in that an adjustment of the damping ratio .zeta. after the unit is mounted on the vehicle is difficult, since the spring coefficient k and the damping factor c of the damper have a fixed value obtained when the vehicle is assembled. It should be noted that the damping ratio is not constant and changes with a lapse of time and use, but it has been impossible in the prior art to obtain a damping effect over a wide range of vibration frequencies.
An improved shock absorber has been proposed wherein a damper is provided with a piston defining an orifice(s) having a dimension which can be varied by a drive means such as a stepper motor, so that the damping factor .zeta. can be adjusted. This improved device, however, suffers from another drawback in that the construction thereof is complex, and the response speed thereof when controlling the damping factor in accordance with changes in the vibration conditions is slow, and thus it is difficult to obtain a desired damping characteristic during a transient state wherein a damping ratio must be quickly changed in accordance with a lapse of time.